1. Technical Field
The present invention relates to the field of integrated circuits, and more specifically relates to amplifiers with a differential output.
2. Discussion of the Related Art
FIG. 1 schematically shows a feedback amplifier circuit 1 having a differential input and a differential output comprising a transconductance amplifier stage 2 having two output terminals “+” and “−” respectively connected to two inputs “+” and “−” of a transconductance inverter amplifier 4. The feedback loop of the amplifier comprises two impedances 6A, 6B respectively connecting output terminals “+” and “−” of stage 4 to input terminals “−” and “+” of stage 2. Two impedances 8B, 8A connect input terminals “+” and “−” of stage 2 to two input terminals (IN+, IN−) of circuit 1. Impedances 8 (8A, 8B) form a voltage divider with impedances 6 (6A, 6B) of the feedback loop. Output terminals “+” and “−” of stage 4 form output terminals (OUT+, OUT−) of circuit 1. Two capacitors 10B, 10A respectively connect input terminals “+” and “−” of stage 4 to its output terminals “−” and “+”.
A voltage divider 12 comprising two identical resistors is connected between the output terminals of stage 4. The midpoint of dividing bridge 12 is connected to a first input terminal of a common mode correction transconductance amplifier block 14 with a differential input and output. A second input terminal of block 14 receives a reference voltage Vref provided by a voltage source not shown. Each of the output terminals of block 14 is connected to an input terminal of stage 4. Block 14 and voltage divider 12 form a non-inverting common mode correction loop.
As the frequency increases, the various amplifier elements of circuit 1 (stages 2 and 4 and block 14) each introduce a phase-shift likely to make unstable, according to cases, the amplification chain of the circuit comprising stages 2 and 4 or the common mode correction chain comprising stage 4 and block 14. Capacitors 10 (10A, 10B), currently called Miller capacitors, ensure the stability of both the amplification chain and the common mode correction chain. The value of the Miller capacitors must be carefully chosen, given that, although Miller capacitors of high value guarantee a good circuit stability, they reduce the gain-bandwidth product of the circuit.
In the case where the differential closed-loop gain of amplifier 1 must be high, the feedback loop, formed of resistors 6A, 8A, and 6B, 8B, is strongly attenuating, which accordingly reduces the open-loop gain of the amplification chain. This attenuation however does not apply to the open-loop gain of the common-mode correction chain. The stability of the common-mode correction chain then cannot be ensured, unless either the gain-bandwidth product of the amplifier is reduced by increasing the value of capacitors 10A and 10B, or the open-loop gain of the common mode correction chain and its correction dynamics are reduced. Such a reduction especially results in reducing the accuracy and enhancing the sensitivity of the common mode correction chain to external disturbances, such as temperature and manufacturing dispersions.